Sewage digestion



Sept. 6, 1938. A. .1. FISCHER El AL SEWAGE DIGESTION Original Filed May31, 1934 8 Sheets-Sheet 1 Wf/f 5. ZU/yp attorney Sept. 6, 1938. A. .J.FISCHER El AL 2,129,266

SEWAGE DIGESTION Original Filed May 31, 1934 8 Sheets-Sheet 2 N545 5. A(/ND WYLVLOLM Sept. 6, 1938. A. J. FISCHER ET AL SEWAGE DIGESTIONOriginal Filed May 31, 1934 8 Sheets-Sheet 3 P 1938- A. J. FISCHER El AL2,129,265

SEWAGE DIGESTION Original Filed Ma 31, 1934 8 Sheets-Sheet s attorneySept. 6, 1938. A. J. FISCHER ET AL 2,129,266

' SEWAGE DIGESTI-ON Original Filed May 31, 1934 Sheets-Sheet 6 E I: 150I163 H 18/ Ff I ma I77 185; a QT) :82

A ZZJ 5. [04 0 5/ attorney Sept. 6, 1938. A. J. FISCHER El AL SEWAGEDQIGESTION Original Filed May 51, 1954 8 Sheets-Sheet 7 Snuentors fCA f%M MA iva-W Gttorneg Sept. 6, 1938. A. J. FISCHER ET AL SEWAGE DIGESTION8 Sheets-Sheet 8 I llll lllllll l l I I l l l l l l 1 I |l.||||IIII-IHHHHHZII:

Original Filed May 31, 1934 Patented Sept. 6, 1938 UNITED STATES PATENTOFFICE SEWAQE DIGESTION Anthony J. Fischer, Jackson Heights, LongIsland,

. and Nels B. Lund, Seaford, N. Y., assignors to The Dorr Company, Inc.,New York, N. Y., a corporation of Delaware 6 Claims.

In the treatment of sewage and analogous polluted waters, according tomodern practice, it is quite common to effect an early separating as bysettling and withdrawal of the suspended solids 5 of the sewage in theform of a sludge and to subject the withdrawn sewage sludge to what isknown as sewage sludge digestion. Sometimes in this class of work thesewagesludge is obtained in a condition which is known as raw oruntreated sludge. According to other procedures either the sewage ischemically treated or the sludge may be treated whereby there may beobtained what may be termed a chemically treated sludge. The chemicaltreatment may have been provided to 15 facilitate the precipitation forthe ultimate obtaining oi the solids as sewage sludge. Also according tocertain procedures the sewage or the sewage sludge is subjected toaeration which is employed as for example to facilitate the precipi- 20tation of the sludge solids called activated sludge or in order toobtain a sewage sludge having certain characteristics which facilitatethe process as a whole.

Raw or untreated sludge often has acid char- 25 acteristics. Where thesewage is aerated the sewage sludge obtained may have an acidcharacteristic particularly where the sludge is stored prior todigestion.

When sewage sludge is digested the acid char- 30 acteristic has to beovercome and the sludge takes on an alkaline characteristic which isdesirable throughout the entire digesting period. This alkalinecharacteristic may be the result of anaerobic bacterial action whichresults in digestion which is duly initiated and which is maintained.

This digestion is the conversion by virtue of bacterial activity ofcertain of the sewage solids into some other physical form such as aliquid or a gas. Some sewage solids are digestible while others are not.Digestible sewage solids are largely putrescible, so their conversion bydigestion thus eliminates their odoriferous nuisance from digestedsewage sludge.

The present invention is particularly concerned with apparatus, methodand/or system for or relating to or useful in connection with thedigesting of sewage sludges or mixtures thereof no matter how obtainedand particularly according to methods in which there is digestion of thesewage sludge as the result of biological or bacteriological actionduring which certain bothersome sewage solids undergo decomposition anddisintegration.

The foregoing remarks have been set forth whereby the general .positionof the present invention is readily oriented in respect to the art ofwhich it constitutes an operative and important part.

The present invention according to one aspect thereof relates to thebiological digestion-01' sew- 5 age sludge and its analogues, and one ofthe objects thereof is to so treat the sludge that its digestion will behastened and so that small digesters of less volume may be used with aconsequent lessening of installation costs while another object is tohasten the production of methane gas due to bacterial action so that thesame proportionate amount of usable combustible gas is derived from thesludge even though the digestion period is shortened.

Further aspects relate to the simplifying of the construction of thedigester; to the reducing of the cost of its operation; to the devisingof means for preventing troublesome scum formation therein; to thecombining of the second stage digester with a gas holder or gasometerand generally to the obtaining of an overflow of liquor of betterquality and of lower solids content than has been obtainable heretofore.Still further aspects relate to the eliminating or avoiding of pumpingof the sludge between digesting stages by automatically displacingdigesting sludge from the first stage to the second stage by raw sludgebeing introduced into the first stage; and to the utilizing of the fullcapacity of the first tank as effective sludge digestion volume inasmuchas no separate liquor storage space or zone need be kept in or aboutthis tank, as is usual in plants asnow operated.

The present invention according to certain phases thereof revolves aboutthe discovery that digester liquor can be depended upon for seeding orinoculating fresh sewage sludge with anaerobic bacteria to initiate andto stimulate the bacteriologi'eal digestion thereof to a greater degreethan can digesting sludge solids, when considered on a drysolids-content basis. Formerly, it was believed that the ratio ofdigested sludge solids to the raw sludge solids was the determiningfactor in calculating the volume of seeding material required. With theaforementioned discovery in mind the process and apparatus of thisinvention is based upon the aim of confining the movement of the heaviersludge solids to a limited horizontal zone while the movement of theliquor is unrestricted and is circulated throughout the entire mass inthe digester; that is, the invention contemplates primarily theselective and differential circulation of digester liquor and digestersolids.

According to another phase of this invention, recognition is given tothe fact that the processes of digestion and sedimentation areantagonistic since sedimentation requires quiescence while digestionrequires turbulence. Therefore, by this invention, it is proposed tocarry out digestion under such conditions that it is not hindered bysedimentation. To that end, digestion is carried out in one container orcompartment while sedimentation is carried out in another. Thusdigestion can be carried out with concurrent conditions of turbulence toinsure homogeneity of the entire sludge mass within the digestingcompartment, while in the other container or compartment any digestionthatmay take place will not essentially interfere with ultimatesedimentation. Thus, one mode of carrying out the invention may be saidto be the rapid stirring or agitating of the sludges while in variousdegrees of bacteriological digestion in a primary digester insubstantially horizontal zones with an additional superposed zone ofnon-horizontal stirring, more specifically of circumnutatory stirring,in a top or upper zone therein. The primary digester has an inlet orfeed and a digesting sludge discharge, but it does not need the usualeiiiuent outlet since sedimentation is normally prevented. Digestingsludge discharged from the digester is conveyed to a sedimentationdevice or clarifler where sedimentation takes place, and digestion istapered off or allowed to taper oil. Settled digested sludge isdischarged from the clarifier along one path, substantially cleareiiiuent is decanted therefrom along another path, and gas generated bythe secondary digestion in rising may-be caught in a gas collectorpreferably associated with or constituting a part of the clarifler orsecondary digester.

The combination of zones of horizontal stirring surmounted by a zone ofnon-horizontal and razzle-dazzle stirring-viz, surmounted by a zone ofcircumnutatory stirring-is an important feature of this invention and sois the feature of a tilted or non-horizontal impeller for setting up therazzle-dazzle action or circumnutatory motion for the purpose of beatingback into-the digesting mass any foamor scum-forming solids and anyliquor that would otherwise tend to accumulate at the top of the mass.The razzledazzle action or circumnutatory motion may be defined asrotatory flow or current movements, the planes of which oscillatevertically. Another feature is the use of impellers or stirrers of theturbo-mixer type which set up fluid currents that are cyclic and to someextent epicyclic.

The present invention according to one important aspect thereof relatesto the digestion of sewage sludge in a system employing multipledigesters or digestion stages, to wit, in a system wherein the larger ormajor part of the digestion of the sludge is carried out in a relativelysmall tank having an associated gas collecting or holding memberconstituting an essential element ofa primary digester, and wherein thelesser part of the digestion is decreasingly carried out and tapers offin a relatively large tank having an associated gas collecting orholding member constituting an essential element of a secondary digesterinto which the sludge which has been almost digested in the primarydigester is transferred for the completion of the digestion and for asedimentation or settling of the remaining sludge solids and into whichgas produced in the primary digester is conducted for storage in thelarger gas holder of the secondary digester.

As previously indicated the digestion which is carried out in thedigesters, being essentially due to anaerobic bacterial activity, isaccompanied with the giving of! or production of an appreciable amountof combustible gas suitable for illuminating, heating and/or powerpurposes and certain aspects of the present invention revolve aboutconstructional and operative features incidental to the handling andcontrolling of the gas thus produced.

The present invention according to one phase thereof is specificallydirected to a sewage treatm'ent-or process according to which raw.activated, chemically precipitated or mixed sludge derived from anysuitable preiiminary'thickening operation-such for example as isobtained from the sedimentation of raw sewageis passed from time to timeto a primary digester wherein digestiondue to the development andmaintenance of active anaerobic bacteria-is carried out in the absenceof air. This digestion is accompanied by the giving ofl or production ofcombustible gas consisting largely of methane-CHr-that is conducted to agas holder provided at the upper part of a secondary digester. Thesecondary digester is connected and arranged with respect to the primarydigester so that as and when incoming sewage sludge is introduced intothe primary digester a corresponding amount of digester treated andmixed sludge, in fact sludge which is almost completely digested, passesfrom the primary digester to the secondary digester whereby the amountof sludge undergoing digestion is maintained constant in the primarydigester. The proportioning of the relative percentage of digestion tobe carried out in the primary and secondary digesters is largely amatter of operative procedure but it is quite feasible for example tocarry out a major part such as approximately of the digestion in theprimary digester and to allow the remaining minor part or 10% of thedigestion to be completed in the secondary digester. The anaerobicdigestion operation is one which is accompanied by rising gas bubbles.These bubbles oppose a tendency for solids to settle and therefore it isadvisable to carry out as far as possible the digestion in the primarydigester and to leave only a small portion of the digestion to becarried out in the secondary digester where sedimentation oi theremaining solids is essentially sought to be obtained.

According to one arrangement of apparatus for realizing the basic aspectof the invention there is employed a primary digester having a fixed orunyielding top-such as is realized by a built-inplace top-and the sludgeundergoing digestion should be considered as approaching close to oreven engaging the under side of the fixed top. However in such fixed topconstruction a receiv in space is provided at the under side of asection of the top for gases developed and given off as a result of thedigestion process carried out in the primary digester. Certainobjectionable features either incident to the mechanical construction orto the operation of the apparatus exist in connection with said fixedtop construction which have indicated the advisability of a verticallyyieldable top which may lift or may float under certain conditions. Theconstruction of such vertically yieldable top involves the solving ofseveral problems before it could be practically employed in order tomeet the several operative conditions to which such a top might besubjected at some time or other during this period of use.

Therefore according to another in certain respects preferred arrangement(or arrangements) of apparatus for realizing the basic aspects of the 75invention there is employed a primary digester (or primary digesters)having a vertically yieldable or vertically movable top or gas holderconstruction. In any event in realizing certain aspects of the inventionthere are employed with the gas holder or top member or cover of theprimary digester-be the top member of the fixed, or yieldable, orvertically movable type-agitating mechanisms known as turbo-mixers, towit, mixing elements which are carried by and supported from the topmember orcover of the primary digester.

In the arrangements shown herein, mixing devices are carried by the topof the primary digester--namely, by the fixed top or the yieldable topas the case may be-and these mixing devices are relied upon foreffecting a more or less constant mixing of incoming sludge with thesludge undergoing digestion and certain aspects of the present inventionrevolve about this construction of the gas holder and particularly aboutthe construction of the yieldable top of the primary digester so that itwill have the required movement. When the alkaline condition essentialfor the carrying out of the sewage digestion by anaerobic bacteria hasbeen once established that condition will not be upset by incoming freshsludge, even though the incoming fresh sludge may have an acidcharacteristic, because the immediate intimate mixing of the incomingsludge through the alkaline body of digesting sludge ensures thepreponderance and prevalence of the alkaline characteristic andanaerobic bacterial activity.

The invention possesses other objects and features of advantage, some ofwhich with the fore-- going will be set forth in the followingdescription.

In the following description and in the claims parts will be identifiedby specific names for convenience but they are intended to be as genericin their application to similar parts as the art will permit. In theaccompanying drawings which constitute a part of this specificationthere have been illustrated the best and most characteristic embodimentsof the invention known to the undersigned but such embodiments are to beregarded as typical only of many possible embodiments and the inventionis not to be limited thereto.

In said drawings Fig. l is a diagrammatic view illustrative of a sewagesystem embodying two sedimentation tanks either or both of which can berelied upon to receive a stream of sewage and from either or both ofwhich at will sewage sludge derived as a result of a settling operationcarried out therein can be passed to a primary digester as operativeconditions permit and wherein the major portion of a sewage sludgedigesting operation is carried out and from which primary digestersewage sludge that has undergone the digestion operation carried outtherein is passed to the secondary digester in an amount correspondingto incoming sewage sludge as and when the latter is passed to and intothe primary digester.

This figure embodies the arrangement of the yieldable top primarydigester and of the vertically movable top secondary digester asconnected up and disclosed in relation to the primary and secondarydigesters of Figs. 6 to 18 inclusive. This Fig. 1 is howeverillustrative of the position of the primary and secondary digesters ofFigs. 2 to 5 inclusive particularly in respect to the clariflers. Thespecific connecting up of the primary and secondary digesters of Figs. 2to 5 inclusive is somewhat different from that of the connecting up ofthe primary and secondary digesters of Figs. 6 to 18 inclusive.

Fig. 2 is a diagrammatic view of a multiple digester arrangementembodying certain basic features underlying the present invention. Inthis figure a primary digester tank is shown as having a stationary topstructure with gas receiving portion provided thereby and this topstructure or top member carries one or more rotary mixing devices orsludge impelling devices particularly designed for efiecting ar'azzle-dazzle or circumnutatory movement at the uppermost portion ofthe body of sewage sludge maintained within the primary digester whilepermitting a lower series of relatively horizontal current movementswithin the lower portions of the body of sewage sludge. In this view asecondary digester is associated with a primary digester. The secondarydigester has a digester tank, which functions as a' sedimentation tank,and a vertically movable gas holder providing an expansible andcontractible gas receiving and storing space. The gas receiving spacesof the two digesters are in communication with each other through themedium of a suitable gas pipe. This figure also shows a conduit leadingfrom the bottom of the primary digester tank to the secondary digestertank, also an efliuent conduit or overflow leading from the secondarydigester tank whereby according. to the particular arrangement shown thehigh portion of the eiiluent conduit for the secondary digester tank andwhich is at the same elevation as the high portion of the conduit,determines the normal minimum level maintained within the primarydigester tank.

Fig. 3 shows in somewhat more detail the primary digester of Fig.2.

Fig. 4 indicates the razzle-dazzle or circumnutatory currents set up bya tilted impeller element designed for that purpose.

Fig. 5 shows a form or sludge impelling or agitating devices accordingto which there are shown in association a tilted impeller element at thetop or upper portion of an impeller shaft and horizontal impellerelements carried by the lower portion of the same shaft.

Fig. 6 is a vertical sectional view showing in operative arrangement theprimary and secondary digesters such as are employed in the arrangementof Fig. 1.

Fig. 7 is a plan view of the primary and secondary digester arrangementof Fig. 6.

Fig. 8 is a vertical sectional view partially broken away showing therelative arrangement 7 and position of certain parts of the primarydigester in respect to certain other parts of the secondary digester andmore particularly the relative position of certain overflowconstructions provided for the primary digester in respect to certainoverflow constructions provided for the secondary digester.

Fig. 9 is a perspective view illustrative of the overflow arrangementsof Fig. 8. In Figs. 6 to 9 inclusive the overflow arrangements for thesecondary digester are shown functioning in a horizontal path 180degrees from the position which the same parts occupy in Fig. 1.

Fig. 10 is a vertical sectional view taken as on the plane indicated bythe line Ill-Ill of Fig. 8

respect to the inlet end of a pipe or conduit leading to the secondarydigester.

' the main features of this gas holder as embodied therein may also beembodied in the gas holder employed in the secondary digester. There isnot included in Fig. 12 the mixing devices carried by the gas holder.

Fig. 13 is a view partially in section showing the mixing device and themanner in which it is applied to the gas holder of Fig. 12.

Fig. 14 is a partial sectional view taken as on the plane indicated bythe broken lines I 4- of Fig. 6 looking in the direction of the arrows.

Fig. 15 is a vertical sectional view taken as on the plane indicated bythe line l5--l5 of Fig. 14 looking in the direction of the arrows.

Fig. 16 is a plan view of an impeller element used in the mixing deviceof Fig. 13.

Figs. 8 to 16 inclusive are shown at a much larger scale than the sameparts are shown in Figs. 6 and 7 and it will'also be noted that Fig. 1is at a smaller scale than any of the other figures.

Fig. 17 shows a primary and secondary digester construction thatfunctions practically the same as the digesters shown in Fig. 6 exceptthat in Fig. 17 the gas holder of the primary digester is shown as afloating cover arrangement that is provided with a pontoon constructionthat defines the peripheral wall thereof. In Fig. 6 each of the gasholders is shown as resting on stops-to wit, a position below that whichthey occupy in normal operation.

Fig. 18 shows a construction wherein primary and secondary digesters areemployed that function much the same as the digesters in the arrangementof Figs. 6 and 17 but in the primary digester of Fig. 18 the normalposition for the yieldable top is that which it occupies when restingupon the stops. In other words the yieldable top of the primary digesterof this figure leaves the stops only due to a decidedly abnormalcondition existing within the digester and in which instance it is freeto yield or move upwardly.

Reference will now be made to the drawings in detail.

Like reference characters indicate parts functioning the same orsubstantially alike wherever they appear.

In the arrangement indicated in Fig. 1 the incoming sewage supply isdesignated by SS. The sewage from this supply is passed at will as alongthe path I to the clarifler, sedimentation tank, or sedimentation basin2 or as along the path 3 to the clarifier 4. Settled sludge is withdrawnat will from one or both clarifiers as along the path 5 or the path 6 asthe case may be and is delivered intermittently by means of sludge pumps1 and/or 8 ultimately along the path 9 that terminates in feed pipe I45to and into the primary digester H constituting part of a multipledigestion system that also comprises the secondary digester tank Illarranged to receive intermittently from the primary digester sludgewhich is undergoing digestion and in an amount which is equal to the fortreatment. This tank I I I and its equivalents,

' while herein are referred to as secondary digesters, are primarily forcarrying out sedimentation and secondarily for carrying out digestion.

Each of these digesters comprises a digester tank and an associated gasholder or gas collector. The tank of the primary digester is designatedby H2 and its associated gas holder or collector by H3. The tank for thesecondary digester is designated by 4 while its associated gasholder orcollector is designated by Hi.

.As previously indicated piping or other passage is provided wherebypartially digested sludge can pass from the primary digester to thesecondary digester as is required incident to the supplying of theincoming sludge to the primary digester. Piping is also provided wherebythe gas receiving spaces within the two digesters are in communicationwith each other during all normal periods of operation. These pipingarrangements will later be described more in detail.

Also as previously indicated the digester arrangement of the system 01'Fig. 1, to wit, the arrangement of the primary and secondary digestersthereof as constructed and connected, is essentially that oi. thearrangements of Figs. 6-to 18 inclusive. The primary and secondarydigester arrangement of Figs. 2 to inclusive can be employed in a systemsimilar to thatoi' Fig. 1 by substituting for the primary and secondarydigesters H0 and Ill of Fig. 1 the primazy and secondary digesters ofFig. 2 and respectively numbered II and 21 and by changing the pipingaccording to the functioning and requirements of the arrangement of saidFigsi2 to 5.

The earliest work in connection with the actual carrying out of certainbasic aspects of the invention is illustrated in Figs. 2 to 5 inclusive.

Therein ll represents the primary digester container or tank foranaerobic biological digestion of sewage sludge which is preferablyround, having a gas-tight top l2 thereon. The top is apertured at l3through which a sludge agitating or impelling mechanism or device may beapplied, fitted with any suitable removable gastight closure i4.Supported from the tank top I2 is a framework i5 from which there dependcertain members or elements of the sludge agitating devices and whichmembers or elements are driven by a motor or engine l6 through appro--priate gearing H. The digester tank H has a sludge inlet l8 and a sludgeoutlet is but no clarified eiiiuent outlet is necessary since the sludgeis agitated or made turbulent to a degree not conducive tof'if notactually preventing, concurrent sedimentation. The tank, preferably thetop member thereof, is also provided with a gas collecting dome or gasreceiving portion and the .tank may also be provided with heating coilsor pipes 2| for insuring desirable temperature conditions in thedigester for anaerobic bacterial activity.

The sludge agitating mechanism III which may be operated intermittentlyor continuously as desired, comprises a rotatable vertical shaft 22preferably mounted oif center in respect to the tank, as shown. Theshaft is rotated from the motor l6 through gearing l1, and carries aplurality of impellers or propellers 23, 24 and 25 constituting sludgeagitating elements. One of the impellers 23 is mounted on the shaft 22near to but below the normally maintained minimum liquid level 26 in thetank in a tilted relationship to said shaft, that is, the plane ofrotation of the impeller is such that it effects a nonhorizontal flow orcurrent movement, to wit, movements which are not at right angles nor ina plane normal to the axis of the vertical shaft 22. The other impellersspaced along the shaft are located in a horizontal position and eifectflow or current movements in planes normal to the axis of rotation ofthe vertical shaft 22. The function of the impellers that arehorizontally mounted, is to agitate the particles of the sludge in thetank in horizontal zones, while that of the top tilted impeller is todiscourage if not to prevent the formation of scum in the zone of theliquid level in the digester by razzle-dazzle or circumnutatory flow orcurrent movements which convectively carry scum particles rising to andwhich would otherwise tend to float and collect at the top of the bodyof digesting sludge to submergence within the sludge whereby the scumparticles ultimately become digested. In this way the collecting andformation of scum is eliminated and avoided. The effect of thisagitation with its consequent hindering of the formation of a floatingscum layer and a deposited sludge layer or blanket is to ensure that theentire sludge mass in the primary digester is brought into a relativelyhomogeneous condition by virtue of which portions of said sludge masswithdrawn from the said digester for further or dewatering treatmenthave on the average substantially the same proportion of liquid andsolid constituents as said sludge mass.

Partially digested sludge from the primary digester II is suitablyconveyed to a secondary digester or digestion zone or tank 21. This tank21 is preferably of the sedimentation type since the function of thistank is primarily to thicken the sludge and to obtain a clarifiedsupernatant therefrom and secondarily to finish the biological digestionof the sludge from the primary digester. The tank 21 is thereforesometimes referred to as a sedimentation tank or basin for in it aremaintained predominant essentially quiescent conditions as required forsedimentation without concurrent predominating conditions conducive todigestion. Accordingly, digesting sludge from the primary digester isfed to the secondary digester 21 through conduit or pipe 28. If thesecondary digester or clarifier is of the Dorr type, which it ispreferred that it should be, then it will have the characteristic Dorrsludge rakes. However, the secondary tank will operate with any sludgeremoval means. This tank 21 is provided with a sludge discharge outlet29, and an effluent overflow 30. The tank may also be provided with agas holder or gasometer 3I to catch any gas generated in the tank.

Where a gas holder is used in association with the secondary digestertank 21, then gas from the gas dome 20 of the primary digester tank IImay be conveyed thereto via pipe line or gas conduit 32 and gas may beremoved from the gas holder 3| by any suitable means such as pipe 33.

The pipe line of gas conduit 32 thus provides a direct line ofcommunication between gas holder 3| associated with the sedimentationtank and the gas dome 20 of the primary digester tank whereby if thesludge level in the container or tank of either is lowered, a gaspressure can be maintained therein to substantially overcome tendenciesof atmosphere to enter thereinto and thus diminish the hazard of anexplosive gas mixture being produced in the container or tank in whichthe sludge level is lowered.

The rotatable shaft 22 of the mixing device results.

should normally be provided with a stuffing box or closure at I4 andhave a steady or step bearing at 35. Clean-outs, such as 36, may beprovided for the tanks II and 21. 31 represents a manhole in digester IIand there may be provided on this digester, a control housing as shownin dotted lines.

Impellers 23, 24 and 25 when of the well known turbo-mixer type haveproved to give satisfactory This type in the form shown has a disc orbody member 39 carrying curved blades or vanes 40 above the disc andother curved blades or vanes 4I below the disc. In the case of thetilted impeller 23, the relative disc and blade construction is the sameexcept that the disc 39, with its blades 40 and M is mounted in a tiltedposition on its hub 42.

In order to assure the agitation in horizontal zones, the arrangement ofFig. 5 may be used wherein horizontal discs 43 and 44 are providedbetween adjacent impellers such as 24 and 25. These discs may be of thesame diameter or of different diameter as shown, particularly the upperone 44 in order to keep the razzle-dazzle action of impeller 23 frominterfering with the horizontal agitation set up by the other impellers.

In the early demonstration plant as to which the showing of Figs. 2 to 5inclusive is illustrative the tank II was 29 ft. in diameter and 15 ft.

deep. The impellers were of the weedless or nonclogging type and 20inches in diameter. They were run at a speed of 120 R. P. M. althoughspeeds up to 600 R. P. M. may be used.

In the operating of said demonstration plant there was an intermittentfeeding of a quantity of previously sedimented sewage sludge to bedigested to the primary digester I I, at or into one elevation such asinto the upper interior portion of the digester II, and a consequentautomatic intermittent displacing of digesting sludge at or from anotherelevation such as a lower elevation, and flow thereof from the primarydigester II through the sludge discharge piping 28 to and into thesecondary digester 21 wherein the anaerobic biological digestion wascarried on to completion. The first or primary digester preferably wasoperated at full capacity so that a minimum of gas collecting spaceexisted between the liquid level and the under side of the gas receivingportion of the digester top.

In the digester I I, the impellers 24 and 25 cause horizontal rotationor agitation of the sludge mass to produce thorough mixing thereof inand throughout the horizontal section or stratum affected thereby. Theresulting turbulence causes an evolution of gas, which in rising to thetop of the digester, tends to form scum adjacent the liquid levelthereof. This tendency to form scum is particularly discouraged if notovercome by the action of the tilted impeller 23 which sets up arazzle-dazzle action or circumnutatory flow current in the digestingsludge as indicated in Fig. 4, and serves to beat back or diffuse thescum particles and any liquor which may rise to the top of the digesterinto the digesting and horizontally agitated mass of sludge.

This type of impeller produced agitation renders unnecessary the use ofa draft tube normally associated with a propeller where mixing effectsare desired. It also stimulates and hastens gas production from thedigesting sludge. This results in asaving in detention time of thesludge in the primary digester I I and this in turn means that asmaller, and consequently cheaper, digester can beused than is nowcustomary. Any gas which may be generated from the secondary digester iscaught by the gas collector thereof and this gas collector isparticularly adapted to serve as a gas storer for the entire digestionsystem. The secondary digester can be made large enough withoutexcessive expense to take care of the winter storage of digested sludge.From the secondary digester, the solids of the digested and settled orthickened sludge is discharged for appropriate disposal. The gas evolvedfrom the digesting sludge can be put to the usual uses for power, heatand light.

Rapid agitation in the first stage increases the rate of gas ebullitionso that when the sludge enters the second stage, most of the gas hasbeen evolved and the rate of gas production per unit of surface area isgreatly decreased. This tends to give a better thickening action in thesecond stage and also gives a clearer overflow liquor as through theeiiiuent overflow pipe than can be obtained in single stage digestion orin two stage digestion where rapid mixing is not employed in the firststage.

Rapid agitation also gives a relatively uniform distribution of solidscontent in and throughout the first tank whereby inoculation ofdigesting sludge, so vital in the proper performance of a digester,cannot be depleted when the sludge from the bottom of the first stage isdisplaced to the second stage or tank by hydrostatic pressure orotherwise. So the agitation with its resulting mixing and minglingtogether with consequent overcoming of Stratification tendencies in thesludge mass or body is for efiecting a substantially homogeneous mixtureof the solid and liquid constituents of the sludge mass. Then it is fromthis mass that fairly representative portions thereof are passed forfurther treatment in the secondary container.

The horizontal type of agitation or agitation in horizontal zones is animportant feature of this invention because it moves the liquor from thesludge without substantially disturbing the solids of the mass beyond alimited horizontal zone, that is, by the use of this type of agitation,a degree thereof can be used which selectively moves the liquor fasterthan the solids therein. The solids appear to have inertia enough towithstand substantial displacement to other horizontal zones by theagitation whereas the liquid particles do not, so that differentialmovement of liquor and solids takes place.

This selective and differential motivation of the liquid particleswhereby they flow past the solid particles is important since it hasbeen found that when used for biologically seeding or inoculating freshsludge for initiating or stimulating the digestion process, the liquorcan be depended upon for greater seeding action on a dry solids contentbasis than can digesting sludge.

Heretofore, it was thought necessary to cause the sludge solids to movethroughout the digester mass in order to get proper seeding, and inorder to produce complete traversal of the mass, the sludge wasstimulated by mechanical means to follow vertical paths up and down inthe digester. Even without mechanical aid, the ebullition of gas tendstoward floating some sludge particles to the top where after release ofthe gas bubbles therefrom, the solids descend through the sludge mass bygravity. However, in spite of all that has been proposed in the past,short circuiting of solids takes place whereby there are digestiblesolids discharged from the digester which have avoided the eifectthereon of the digestionprocess. Also, in prior digester practice ascommercially canried out, sedimentation takes place which results in thesludge solids settling to the bottom of the digester and the liquorrising to the top thereof as supernatant. The sedimented sludge in saidbottom becomes more or less concentrated or thickened and immobleresulting in the dying oif of innumerable micro-organisms, so that thesupernatant is not available to seed -biologically the undigested butdigestible solids remaining in the digester particularly above the zoneof sedimented digested sludge.

These disadvantages are obviated by the use of this invention. Since itis now proposed to make use of the circulation of digester liquorundergoing digestion for seeding instead of the circulation of solidswhich have become digested or substantially so, it becomes unnecessaryto stimulate the circulation of the latter so they can be restricted toa limited zone of movement. This restricted movement of the solidsprevents the former trouble of short circuiting. The completecirculation of the liquor while producing limited circulation of thesolids is accomplished by the horizontal zones of agitation set up bythe impellers, as above described, namely, by causing the liquor to movefaster than the heavier solids. The solids tend to remain in thehorizontal zone where they find themselves whereas the liquor traversesthe entire digester mass, which traversal is aided by the razzle-dazzleand beating back action of the tilted top impeller. Thus by persistentand unrestricted movement of the liquor it is maintained in an optimumcondition for both the maximum growth of favorable micro-organisms andthe maximum seeding capacity thereof because the liquor disperses themicro-organisms throughout the digester mass without substantiallydistributing the solids beyond their horizontal zones.

It will also be noted that sedimentation in the primary digester l I isendeavored to be prevented because sedimentation and digestion are twoantagonistic processes. Sedimentation requires quiescence while forrapid digestion turbulence is required. Consequently by this inventionthese two processes are kept separate with that of digestion essentiallytaking place in the first tank and that of sedimentation essentiallytaking place in the other tank. More particularly, in the primarydigester is caused to prevail optimum conditions of turbulent sludgemixing and mingling for burnable-gas-producing bacterial digestion ofthe sludge without concurrent quiescent conditions conducive tosedimentation, and sludge having been treated under such conditions isthen passed as a mixture that is fairly representative of the digestingsludge mass to a decanting sedimentation container so separated from thedigester container that the operative conditions in one container do notinfluence the operative conditions in the other, in which sedimentationcontainer there are maintained essentially quiescent conditions asrequired for sedimentation without concurrent turbulent conditionsconducive to gasifying bacterial digestion.

In normal operation of the system of Fig. 2 the liquid or sludge levelis normally maintained up to minimum by the high section or elevation oithe pipe 28. Should theoverflow pipe 30 of the secondary digester be ashigh as or even higher than the high section of 28 then said overflowpipe might be considered as determining the minimum level maintainedinthe primary di-' gester tank. This statement would hold ex- 2,199,96dcept for such periods as, the level in the secondary tank 21 istemporarily lowered .incident to the withdrawing 'ofsludge therefrom andin which instance the minimum level within the primary tank II would bedetermined by the high section of the conduit 20. The control of theminimum level of the sludge mass or body in the digester by the highsection of the pipe 20 is important because the amount of sludge in thedigester to which fresh sludge is intermittently fed is thus assuredlyrelatively large enough to continue therein apredominance orpreponderanace of anaerobic bacteria as evidenced by the production ofcombustible gas.

It will be noted that by the arrangement of this plant, as well as bythe arrangements hereinafter specifically described, a construction isprovided which readily lends itself to the placing of the primary andsecondary tanks whereby the bottoms thereof are at approximately thesame general level or elevation thus avoiding any undue excavation forone tank as compared with the other.

Reference will now be made to later or other developments wherein novelyieldable or movable top constructions for the primary digester aredisclosed and wherein other novel features and details are alsoembodied. The arrangements of said later or other developments arecommercially practicable in carrying out or realizing the particularinvention herein sought to be protected and hence a description of saiddevelopments is retained herein. It is to be understood that theessentials of the invention herein sought to be protected and thefundamental functioning features thereof as above described apply withequal force and effect to the construction and operation of the form ofapparatus herein described in detail. The detailed description of theapparatus of said later developments is included herein in order thatthere may exist the benefits incident to that full and comprehensivedisclosures which is indicative of different forms and ways in which theinvention may be realized or performed.

The construction of the associated digesters of Figs. 6 and 7 will nowbe described and more particularly in. conjunction with reference toFigs. 8 to 14 inclusive. The tank H2 of primary digester H is made ofconcrete. It comprises a floor or bottom portion H6 and peripheralupstanding wall Ill the latter of which carries at the upper interiorportion thereof an inwardly extending stop ring or corbel I09 providinga stop for the gas holder or gas holder construction H3 and forsupporting the .latter in a lowermost position therefor at the upperinterior portion of the tank. This stop ring or corbel I09 serves toprevent the loss of gas through the liquid seal around the holder bydeflecting rising gas bubbles from the liquid sealing section. The tankH2 is also provided with a vertically extending centrally located guidepost H0 which ,is fixedly secured at H9 relative to the floor of 'thetank structure.

The gas holder H3 is provided by a vertically yieldable and in fact avertically movable structure which comprises a sheet metal top I20 and adepending peripheral wall I2I, a vertically extending centrally locatedstrut I22 and downwardly inclined connecting members I23 which extendfrom the peripheral wall I 2I inwardly and downwardly to a place wherethey are connected to the vertically extending strut member I22. Thisvertically extending strut member I22 is sliding engagement with theguide post H0. See in this connection the arrangement in Fig. 14 whereinthe post H8 is shown as carrying antifriction rollers I08'that engage aninterior section of the strut or guide tube I22. This strut member I22has a dualfunction, it serves in conjunction with guide post H8 to guidethe vertical movement .of the gas holder H3 and to ensure that the topI20 thereof shall maintain a desired horizontal arrangementfor allpositions thereof. It also serves to reinforce and support the top I20when the gas holder is resting in its lower: most position. Thisprovides a construction for preventing the top I 20 from buckling,bending or collapsing incident to external air pressure exerted on thegas holder.

The top member or portion I20 of the gas holder is in the form of anumbrella cover or inverted saucer. More specifically and more accuratelyit may be described as preferably in the form of a top section of ahollow sphere, the periphery of which section is in plan a true circle.

The gas holder however need not be circular in horizontal cross sectionas any other geometrical form which in horizontal cross section presentsa symmetrical arrangement could be employed as for example one showing ahexagonal, octagonal or other symmetrical form. Moreover the top memberor top construction of the gas holder need not be in the form of asphere top, of an inverted saucer, or resembling that of an umbrella topsince any suitable geometrical form couldbe employed, be it that of acone, of a pyramid, a frustrum of a cone or a frustrum of a pyramid. Anessential factor is that the top member or top construction of the gasholder shall be hollow so as to have a concave portion at the under sideand which is well constructed to withstand either internal or externalpressures as and when applied thereto.

The peripheral wall I2I of the top holder depends downwardly from saidconcave top section or inverted saucer shaped. top member. Thisperipheral wall may be defined as a peripheral ring or member which hasa gas-tight connection or union with the top member whereby a gas-tighttop construction ior the holder is provided. This peripheral ring orwall I2I is of metal and is loaded with concrete ballast II". Thisconcrete ballast is relatively heavy and the function thereof is toplace a relatively progressive and increasing load on the gas holderparpreterably tubular in form and has a vertically ticularly as the gasholder rises sufficiently to lift the concrete ballast so that itprojects above the sealing liquid and whereby a progressively andsubstantially increasing load is imposed upon the movable holderaccording to the amount of ballast projected above the liquid level. Inthe lowermost position of the gas holder the lower edge portion of theperipheral wall or ring I2I rests upon the stop ring or corbel I09. Aninspection of the drawings will make it clear that this stop ring ispositioned so that the gas holder ment and so that the gas holdingcapacity thereof can be relatively large.

The strut member I22 heretofore described as a tubular member isprovided with openings I06 at the top end thereof whereby any pocketingof gas that might otherwise affect the movement of the gas holder isavoided.

Mixing devices or sludge impelling mechanisms 24, frequently referred toas turbo-mixers, are carried by the gas holder so as to be verticallymovable therewith. The arrangement of these mixing dev ces is clearlyshown in Figs. 6 and 7 and the general construction thereof and mode ofmounting the same is clearly shown in Figs. 13 to 16 inclusive. Themixing device (or de-.- vices) 24 is similar in essential operativecharacteristics to the mixing device or sludge impelling mechanism I0previously described in connection with the system of Figs. 2 to 5inclusive. Each of these mixing devices I24 comprises a base I 25 whichis detachably secured to the top I20 as through the medium of bolts I26and in a manner whereby a gas-tight joint is provided as between the topI20 and the base I25. Upon this base I25 there is carried the housingI21 of an electric motor I28 the rotor of which is connected so to drivethrough the medium of suitable gearing and coupling I20, a verticallyextending shaft I30 of the mixing device so as to effect the normalrotary movement of the latter. Suitable gas-tight packing I3I isprovided below said coupling I 20. In the immediate region of eachmixing device a depending ring I32, which is circumferentially arrangedwith respect to the shaft I30, depends from the cover a suflicientdistance to extend into the liquid or digesting sludge maintained up toa minimum level within the primary digester when the latter isfunctioning. The pu p se of this depending ring I32 and liquid sealresulting from the use thereof is to provide an arrangement whereby, atwill, the mixing de-.

vice as a whole can be removed from place without allowing the escape ofgas from the holder by merely'removing nuts from the bolts I26.

Each mixing device is provided with non-clogging impellers as I33 invertically spaced arrangement. Each of these impellers comprises a hubsection I34, a body section I35 arranged in an inclined position inrespect to the hub axis whereby when the impellers are in place thebodies are tilted at a slight angle from the horizontal and eachimpeller also includes vanes or blades I36 for producing impellereffects in directions away from the axis. The inclination of the body asit rotates produces vertical components of flow movements while thevanes 0r blades produce horizontal components of flow movements and thefunctioning of these impellers produces what may be referred to as therazzle-dazzle or circumnutatory motion or mixing action within theliquid body affected thereby. The functioning of these impellers, whichare also offset or eccentrically located in respect to the general axisof the tank as is manifest from Figs. 6 and 7, produces what may begenerally termed epicyclic type of currents or movements throughout thevarious sections of the liquid body.

Some or all of these impellers are frequently made and employed so thatthe body section thereof need not be inclined to the hub axis with theresult that the body section and the impellers move in true horizontalpaths thus avoiding most or much of the vertical components which areexperienced where the body is inclined to its axis. Impellers withbodies at right angles to their axes are useful in the lowermost layersof the sewage sludge but it will be pointed out that the impellers withthe bodies inclined to their axes of rotation are particularly useful inthe upper layers of the sludge since the operation thereof assists inbreaking down scum and in the thorough mixing of the tank contents insuch a manner that the accumulation of scum at the upper interiorportion of the primary digester is avoided due to scum particles beingswept by the razzle-dazzle currents back to submergence within the bodyof digesting sludge.

The mixing devices as illustrated herein have been described inconsiderable detail and as previously indicated function as non-cloggingimpellers or in other words are of the non-clogging type. It is feasibleto construct each impeller so that it primarily comprises or consists ofa body or hub portion and blade members, which bothr or hub portion issecured to the vertical extending propelling shaft to which the impellercorresponds and which blade members extend outwardly from said body orhub portion. Such blade members may be considered as compositely definedby the upper and lower blade elements heretofore described. The bladeelements function to effect a forced or circulatory movement of liquidaway from the shaft particularly since there is no peripheral restraintagainst outward flow because thereis not any ring or other restrainingmember located at or in the immediate vicinity of the periphery of theimpeller.

As to the impellers herein shown some have body members in effectprovided by plates that extend at right angles to the axis while somehave body members in effect provided by plates that are inclined to theaxis about which they are rotated. It is feasible, however, to constructimpellers that will function along the same lines as the impellersherein illustrated and this even though the plate portions just referredto are not necessarily employed. By bearing in mind the particular workto be done by a particular impeller it is merely a matter of design toconstruct the impeller to do the work desired therefor.

The lower ends of the mixer shafts I33 are provided with removablesteady bearings each of which includes a bearing member I3I that isretained upon the shaft I30 and which is removable therewith because ofspaced collars I38I30 which are secured to the shaft. In this connectionsee Figs. 13, 14, and 15. Each bearing member I31 has an outer sectionI05 in the shape of a frustrum of a cone whereby it readily sets orpositions itself in place on the conical seat section I39 which iscarried at the outer end of the corresponding extension arm constructionI40 that includes parallel channels I and I42. The intermediate sectionsof these channels are secured in any suitable manner as by welding atI43 to the vertically extending strut member I22. Braces I44 are reliedupon for firmly supporting the outer ends of the arm construction I43against downward deflection. It will be noted that the outside diameterof each impeller I33 and of the removable steady bearing member I3I isless than the inner diameter of the depending ring I32 with the resultthat the mixing device I24 as a whole can be readily removed andreplaced at will solely by the operator removing nuts from the bolts I26and vertically moving the mixing devices from or towards its normalposition in respect to the gas holder top.

The primary digester receives the sewage sludge to be digested in anysuitable manner as through the feed pipe I45 which ,it will be notedaccording to the arrangement shown delivers the incoming or influentsewage sludge into the upper interior portion of the primary digestertank, preferably through a section of the feed pipe I 45 which islocated somewhat below and which extends past the supporting surface atthe top of the stop ring or corbel I08.

Sewage sludge, which has been treated, passes from the primary digesterthrough a discharge section I46 into a discharge pipe or conduit I"having a valved branch I48 and an upwardly extending branch I49 thelatter of which terminates in an adjustable overflow or weirconstruction I50 that is relied upon for ensuringthat the minimum levelof the body of sewage sludge retained in the primary digester shallalways be at least up to the height determined by the overflow. For thedetails of the overflow arrangement of both the .primary and secondarydigesters see particularly Figs. 8 to 11 inclusive.

In the normal functioning of the primary digester'the valved branch I48is closed whereby all of the sludge passing from the primary digestermust pass through the overflow branch I49. It will also be here notedthat as sludge is supplied to the primary digester for treatment thereina corresponding amount of sludge which has undergone digestive treatmentin the primary digester passes therefrom through the overflow branch I49on its way to a secondary digester as will hereinafter more clearlyappear.

The primary digester also has a gas ofi'take piping or conduit II theupper intake end I52 of which extends into the gas receiving space whichis always maintained within the upper interior portion of the primarydigester, to wit, into the gas receiving space at the under side of thegas top I20 on the one hand and the top of the body of sewage sludgemaintained within the primary digester on the other hand. This gaspiping I5I extends into the secondary digester and terminates in a pipesection I53 the upper end of which extends into a gas space provided inthe upper portions of the secondary digester with the result that duringthe normal functioning of the digesters the gas spaces of both digestersare, in constant communication. The upwardly extending delivery endportion or pipe section I 53 derives support from the floor of the tankof the secondary digester through the medium of supporting rod I532,

The primary digester is also preferably .prO- vided with a normallyclosed depending pipe I54 (see Fig. 6) that always dips into the sludgewithin the primary digester but which is arranged to provide a samplingopening by which ready access for sampling purposes is realized.

The primary digester is provided with heating coils I55 which are usefulin ensuring suflicient heat to the sewage sludge to facilitate thedigestion that is carried out therein. The gas holder is secured againsthorizontal rotation relative to the tank through the medium of chainsI56 and I 57 which are referred to as tangential guides.

The secondary digester III is shown larger than the primary digester andcomprises a secondary digester or sedimentation tank II4 provided with adownwardly and inwardly inclined bottom H6 and upstanding peripheralwall II'I carrying spaced stops I6 I. The secondary digester has avertically yieldable or vertically movable gas holder II5 that comprisesthe umbrella shaped top sections I63 from which there depends theperipheral wall I64 preferably carrying at the lowermost edge thereof aring of concrete ballasting material I65. The peripheral wall I64 of thegas holder when the latter is in lowermost position rests upon the stopsI6I. The general construc tion of the gas holder of both digesters issubstantially the same in that each comprises a centrally locatedvertically extending strut as I22 for the-primary digester or I66 forthe secondary digester and inwardly and downwardly extending tie rods orconnecting members as I23 for the primary digester or I61 for thesecondary digester, which rods or members serve to support the lower endof the centrally located strut corresponding thereto. The lower edgeportions of the peripheral ring I64 are supported against inwardcollapse by radial strut members I62 the inner ends of which rest uponand are secured to the strut I66. The strut I66 in the secondarydigester is also a tubular member having vent holes I58. The guidemember or tube I66 slides along a fixed vertically extending guide postI68 which is positionably secured in place in respect to the masonryproviding the floor of the secondary digester tank. By this constructionthe secondary gas holder is guided throughout the entire range of itsnormal vertical movement and the strut construction also serves tointeriorly support the plate top section of the digester againstinternal buckling or collapsing should there be realized a decrease inpressure within the tank and an excessive unbalanced pressure on theoutside of the tank.

Reference has heretofore been made to the valved branch I48 of theprimary digester. This branch is connected to a T or cross pipe fittingI69 leading for example to a sludge pump as along path I00 or to a placefor other mode of sewage solids disposal as along the path SB to asludge bed.

The tank of the secondary digester has a solids discharge section Ileading to a valved sludge pipe or conduit III which terminates in a Tpipe fitting I 69. It will therefore be apparent that the piping whichincludes the pipe I 47, cross fitting I69 and pipe I'II when the valvesare open can be employed for the direct passage of sewage sludge fromthe primary digester to the secondary digester. Normally the valves areclosed so this direct transfer cannot take place. Upon the opening ofeither the valved pipe I48 or the valved pipe I'II it will be manifestthat sludge can be pumped at will from the primary digester or secondarydigester to and through the cross fitting I69, thence along the path I00by a pump as I or 8. See Figs. 9, 7, 6 and-1. It would also be possibleto efiect a reverse pumping action to unplug the valved pipes I4'I orI'll should occasion require. The secondary digester normally func tionsover an extended period without the withdrawing of sludge and then atthe proper time the deposited solids remaining after the digestion andsedimentation carried out therein are removed through the pipe "I. Thisremoval of sludge solids of course efiects a lowering of the normalwater level within the secondary digester and this is accompanied by alowering of the gas holder. In order to avoid lowering of the liquidwithin the secondary digester in such a manner as to unduly reduce thegas pressure therein whereby too great a differential as between theexterior air pressure and the internal pressure will take placeso greata difierential as will tend to cause the gas holder to collapse orfail-there are provided certain ordinary check arrangements, as follows:a sludge check or ball valve I12 that is carried by chains I13 isprovided and' which sets itself in the solids discharge section I10slightly before or at least by the time that .lowermost position withthe result that communication between the digesters is shut off and alsowith the result that further withdrawal of gas from the secondarydigester cannot take place. A sealing ring I60 depends from the topmember I63 of the gas holder for the secondary digester (see Fig. 8.)This sealing ring dips into the sludge liquid within the secondarydigester when the gas holder is in its lowermost position and providedthe liquid is up to the minimum height as determined by the overflowweir construction for the secondary digester. A sealing ring such as I60and which depends from the cap portion or top member I63 0! thevertically movable gas holder may be viewed as part of an inverted cupshaped construction provided for the reception of the upstanding portionI53 of the gas piping or conduit providing means that extends from thegas-receiving space of the primary digester to the gas-receiving spaceof the secondary digester. The structure providing the inverted cupshaped member construction constitutes what may be referred to as aninverted cup shaped member that is completed by a removable cover or topportion I93. This removable cover I98 may be viewed as a manhole or handhole cover and which when removed permits access to the interior of thecup and to a chain or other flexible connecter carrying the valve I14(see F18. 8) or even to a flexible chain or connecter such as I13 towhich the valve or plug I12 is connected. It will also be noted that themember I60 may be viewed as an inner flange depending from the capportion of the vertically movable gas holder. The cup shaped member isnormally closed except at the bottom whereby the receiving space isprovided for the upstanding end I53 of the gas piping. The lower end ofthis flange or inverted cup is at an elevation substantially below thatof the underside of the cap portion on the one hand but substantiallyabove that of the lower edge of the marginal flange on the other. Asindicated herein when the gas holder is in the lowermost position itcauses the sealing ring or depending inner flange I60 to dip into thesludge body sufficiently to shut off the flow of gas past and below thesealing ring and then into and through the upper end of the gas piping.Under such conditions access to the valve I14 is afforded withoutallowing the escape of gas from within the secondary gas holder.Likewise a similar sealing ring I59 is provided at the under side of thetop portion of the gas holder for the primary digester. The sealing ringI59 dips into the sewage sludge when the gas holder is in its lowermostposition. Under the conditions specifled--with the gas holders in theirlowest position-the flow of gas therefrom into the gas pipe I5I isprevented.

It has heretofore been pointed out how the sewage which has undergonetreatment in the primary digester passes therefrom by the overflow I50as sewage sludge to be treated is supplied to the primary digesterthrough the feed pipe I45.

An inspection of Figs. 9 to 11 inclusive will 'make it clear that thedigesting sewage sludge which passes the overflow I50 enters thereceptacle or trough I16 and flows therefrom through a pipe I11'thatdelivers the sewage to the secondary digester and more particularly intothe lower section of the liquid body maintained within the secondarydigester.

These figures also show a second overflow pipe or conduit I18 leadingfrom the primary digester and terminating in the adjustable overflowsection I19 that is somewhat higher than the overflow I50. This secondoverflow, I18 and I19, is provided in case the overflow branch I49 whichterminates in overflow I50 becomes plugged or unduly loaded, therebyavoiding any undue excess of liquid level within the primary digesterduring the normal operation thereof.

The secondary digester has an overflow construction relatively remotefrom the place where sewage sludge is introduced into this secondarydigester as will be seen from an inspection of Figs. 6 to 13 inclusive.The overflow arrange ment for the secondary digester comprises anofftake or outflow pipe or conduit I leading from below but near theupper normal water level for the body of liquid within the secondarydigester as is determined by the overflow arrangement for this digester.This pipe or conduit I80 leads through the wall of the digester andterminates in an adjustable overflow or weir section I8I for determiningthe maximum normal water level within the digester. The upwardlyextending intake end section of the pipe or conduit I80 derives supportfrom the floor of the tank of the secondary digester through the mediumof supporting rod I80. In case this pipe construction I80 or I8I shouldbecome plugged or ineffective there is provided an auxiliary overflow oroutflow pipe I82 that terminates in an adjustable overflow or weir I83the latter of which is some- 1 what higher than the overflow I8I. Forconvenience and comparison the overflow arrangements of the primary andsecondary digesters have been brought into close relationship in Figs. 8and 9. overflows-II" and I83 of the secondary digester deliver into atank or receptacle I84 from the bottom of which there extends a pipe orconduit I85 for conducting the relatively clear delivered eflluent tosuch place of use or disposal as is deemed desirable. The normallyfunctioning overflow -I5Il for the primary digester occupies a higherelevation than the normal overflow I8I or even the overflow I83 for thesecondary digester and it will be apparent that when the liquid levelwithin the secondary digester is up to the normal maximum level asdetermined by overflow I8I that a feed of sewage into the primarydigester simultaneously effects/a flow of a corresponding amount oftreated sewage from the primary digester into the secondary digester anda corresponding flow of relatively clear eilluent from the secondarydigester past the overflow I8I thereof. The secondary digester ispreferably provided with sampling pipes I86, I81, etc., whereby samplesfrom the various levels within the secondary digester can be readilyobtained. The secondary digester has any suitable sway bracing andguides such as are provided for example by sway chains I88 and I89 andby the vertical guideways indicated at I90.

In certain localities where freezing may take place arrangement is madefor employing a removable seal heating coil pipe as at I9I for theprimary digester and as at I92 for the secondary digester wherey thesealing liquid or sludge around 75 the gas holders will not becomefrozen and thus prevent a moving or yielding of'the holders. Each gasholder is also preferably provided with a normally closed manhole suchas indicated at I98 for the primary digester or at I94 for the secondarydigester.

The combination of primary and secondary digester as just described indetail particularly as shown in section in Fig. 6 typifies theconstruction of the primary and secondary digesters employed in thesystem of Fig. 1. A somewhat better or more comprehensive understanding,however, is obtainable upon a further consideration of the arrangementand operation of the parts as outlined in Fig. l and therefore a moredetailed statement in respect to said figure is now set forth.

The reference character V which appears in several places on thedrawings indicates a valve which can be opened or closed to permit aflow therethrough or to shut off a flow past the same according to thedesires of the operator.

Water supply pipes are indicated on the system by W and are providedwhereby water is available if desired for any particular purpose.

WL indicates the normal minimum liquid level within a particulardigester as determined by the overflow or weir construction associatedtherewith.

As previously stated the sewage supply to be treated passes from SSalong a path I or 3 as the case may be, to a clarifier or thickener 2 or4 wherein the sewage sludge is settled out and from which the sewagesludge thus settled out it withdrawn for passage to the primary digesterH0. It is well within the contemplation of the present scheme thatchemical treatment or even aeration of either the raw sewage prior tosedimentation in the clarifler 2 or 4 or of the sewage sludge therefromcan be carried out.

The eiliuent from the clarifier is passed therefrom as along the path 2Aor 4A for such disposition as may be desired for a particular plant. Anyscum collecting at the top of the clarifier may be passed therefrom asalong the path 28 or B to a point where it will join the sludge from theclarifier.

Sludge pumps I and are connected in the piping system that includes thepipe lines and 6 whereby at will either one of the pumps can be operatedto function in the pumping of sludge from either of the clarifiers orwhereby if desired one pump can be pumping from one clarifler while theother is pumping from the other clarifier or whereby one of the pumpsmay be pumping from one clarifier while the other of the pumps may bepumping sludge solids from the secondary digester as will hereinaftermore fully appear.

Either or both of the pumps 1 and 8 can be relied upon as desired topass sewage sludge into the primary digester through the feed pipesection I45 thereof or at will either or both of the pumps can deliverliquid pumped thereby along the path 93 for such use or disposition asdesired or as along the path 9A as for example to a sludge receiving bedalong the path marked SB.

From Fig. 1 it will be seen that the suction side of either or both ofthe pumps 1 and B can at will be connected through the medium of pipeline I00 in a manner to draw or pump sludge solids from the secondarydigester III or even to pump sludge from the primary digester H0.

In the normal functioning of the system this line I00 is closed and thenormal functioning of the pump or pumps then is to deliver raw sewagesludge into the primary digester thereby forcing or else causing afeeding of sludge which has undergone digestion from the primarydigester into the secondary digester and, providing the liquid level inthe secondary digester is up to the minimum level to be maintainedtherein, there follows a flow from the secondary digester of relativelyclear efiluent which can be passed along the path I 05 and branch I02 toone place for disposition or as along the branch I03 back into the rawsewage supply. If, however, it is desired to withdraw sludge from thebottom of the secondary digester either of the pumps can be employed forthis purpose by opening the proper valve and the withdrawn sludge can bepassed from the system as along the path 93 or 9A as desired and. thiscan take place even though the other pump is functioning to supply rawsewage to the primary digester tank.

The arrangement whereby sewage sludge can be pumped from either of thedigesters H0 or III through the pipe line I00 is clearly shown in Figs.6, 7 and 9 and comprises the pipe I41 and valved control pipe H8 leadingto the cross member I69 and the valved control pipe I'll also leading tothe cross member I69. The gas piping or conduit which includes the partsI52, I5I and I53 previously referred to (and in this connection seeFigs. 1, 6, 7 and 9) places thegas receiving portions of the primary andsecondary digesters in communication and it includes a condensate trapCT whereby accumulating water may be eliminated from the gas line. Thegas will be employed for heating by passing the same along the path I04through condensate trap CT past meter M to the furnace of a hot waterboiler 205 or directly to burners for lighting or other use as along thepath 206. A pressure relief trap may be provided at 201. The hot water,boiler includes a heating coil I-IC which is in a circulator systemthat includes the heating coils I55 of the primary digester and whichmay also include the removable heating coils ISI and I92 provided forpreventing any freezing of the seals in the primary and secondarydigesters. The circulation may be maintained as by means of thecirculating pump 208. In Fig. 1'? there is shown an arrangement ofprimary digester and secondary digester each having a tank andassociated vertically movable gas holder or top constructed so that thetop for the primary digester has a relatively limited movement while thetop for the secondary digester has a relatively long vertical movementwhereby the latter can function as a gas storer as well as a gascollector. The functioning of the arrangement of this Fig. 17 followsvery closely the functioning of the arrangement shown in Figs. 6 and 7and therefore any extended description in reference to Fig. 17 is notnecessary. The gas holder, however, of Fig. 17 is designated as 2I0 andit will be seen that there is associated with the sheet metal top orroof portion 2I3 thereof a buoyant peripheral ring provided by thehollow gas-tight annular section or pontoon 2 which is defined by theouter portion 2I5 of the top 2I3, the lowermost annular portion 2I6, anupright outer ring 2H and an inner ring 2I8. In Fig. 17 the gas holder2I0 is shown resting on the stop ring or corbel I09 but in the normalfunctioning of the digester this cover occupies a slightly elevatedposition approximately that indicated by the dotted lines. It will bereadily appreciated that this cover can have a slight vertical movementincident to any fluctuations of liquid level within the tank, also inresponse to variations in gas pressure conditions, but that the pontoonconstruction provided by the hollow annular section 2 described issufficiently heavy to effect a material loading of the top whereby anyextended lifting movement of the top relative to the liquid and incidentto fluctuation in gas pressures during normal operation is avoided.

In Fig. 18 there are shown a primary digester and secondary digestersimilar in many respects to the arrangements shown and described inconjunction with Figs. 6 and '7. According to the construction of Fig.18, however, the gas holder of the primary digester is verticallyyieldable but it is loaded so as to remain during the normal operationof the digester in the lowermost position therefor, to wit, at rest uponthe stop ring or corbel I09. In this figure the gas holder as a whole isdesignated as 3l0. It comprises the umbrella shaped sheet metal topmember or roof portion 3l3 and has a peripheral ring Ill, part of whichring depends from and part of which ring extends above the peripheraledge of the umbrella shaped portion M3. At the exterior top peripheralportion of this gas holder or top member 3 i 0, as at 3l8, there isplaced suitable weighting material as concrete which ensures weightsomewhat in excess of that required for normally holding the cover inlowermost position on the stops and which functions in opposition to anybuoyant effect of the sludge liquid within the tank H1 and against'thenormal gas pressures experienced within the digester. Otherwise than asindicated the arrangement of this Fig. 18 operates substantially alongthe lines described in connection with the arrangement of Figs. 6 and 7.

This patent (hereby identified as Case 3) is a division of applicationSerial No. 728,376 flied May 31, 1934 (hereby identified as Case A) andmaturing on January 29, 1935 as Patent No. 1,989,589.

Said application 728,376 is a continuation in part of our patentapplication Serial No. 680,450 filed July 14, 1933 in which application680,450 Figs. 2 to 5 hereof appeared with a description thereof and withcertain subject matter claimed to which the claims of the presentapplication is directed. Said application 680,450 has been dropped (butnot the invention thereof) after the filing of the said applicationSerial No. 728,376 (Case A). As to all of the subject matter of thisdivisional patent (Case 13), we claim an effective filing date at leastas early as May 31, 1934, to wit, the filing date of Case A, and as toany and all subject matter common to this patent (Case B) and to saidapplication Serial No. 680,450 we claim July 14, 1933 as the effectivefiling date, to wit, the filing date of said application Serial No.680,450.

We claim:

1. In a multi-stage sewage sludge digestion system a. primary digesterand a secondary digester each comprising a sludge holding tank and a topmember at the underside of which there is a gascollecting space and ofwhich digesters at least one top member is essentially provided by avertically movable gas holder having a cap portion and dependingmarginal flange dipping into the sludge body within the tank whereby theresulting gas-collecting space is expansible and contractible; means forfeeding sewage sludge into the primary digester; sludge transfer meansproviding a conduit leading from the interior of the primary digester tothe interior of the secondary digester and having an overflow sectionfor denormally fail; liquid withdrawal means leading from the secondarydigester having an eifluent overflow section that determines a levelbelow which the top surface of the sludge body within the secondarydigester does not normally fall: and means providing a gas conduitextending from an elevation within the primary digester that is higherthan that of the overflow section of the sludge transfer means, thencelaterally beneath and past the lower edge of the marginal flange of theaforementioned vertically movable gas holder and thence upwardly intothe gas-collecting space within the secondary digester whereat itterminates at an elevation higher than that of the eilluent overflowsection of the liquid withdrawal means.

2. In a multi-stage sewage sludge digestion system as defined in and byclaim 1 an inverted cup shaped structure carried by the movable topmember, disposed for the reception of one end of the means providing thegas conduit, andof which the open low end is at an elevationintermediate the underside of the cap portion of the vertically movablegas holder and the lower edge portion of the depending marginal flangethereof.

3. A sewage sludge digester comprising a sludge holding tank, avertically movable gas holder having a cap portion and a dependingmarginal flange extending downwardly into the body of sludge within thetank; means for feeding sludge into the tank; liquid withdrawal meansleading from the tank having an efliuent overflow section fordetermining the normal level of the sludge body within the tank, andmeans providing a gas conduit extending from an elevation within the gasholder that is higher than the said eilluent overflow section and thencelaterally below and past the depending marginal flange; which digesteris characterized in that the vertically movable gas holder carries aninverted cup shaped member normally closed except at the bottom anddisposed for reception of the upper end of the means providing the gasconduit, the lower edge of which cup shaped member is located at anelevation intermediate that of the underside of the cap portion of thevertically movable gas holder and the lower edge of the dependingmarginal flange thereof.

4. A sludge digester as defined in and by claim 3 having a flexibleconnecter secured to the ver tically movable gas holder and a valvemember secured to the flexible connecter, and according to which theinverted cup shaped member has a section providing a sealing flangedepending from the cap portion of the vertically movable gas holder anda cover section which is removable, thereby permitting access to theflexible connecter.

5. A sludge digester as defined in and by claim 3 according to which theinverted cup shaped member provides a sealing flange extendingdownwardly from the cap portion of the vertically movable gas holder,according to which the inverted cup shaped member has a removable coverplate for permitting access to the interior thereto, and according towhich a valve member movable with respect to the gas holder but carriedtherefrom is positioned so as to close the passageway through the gasconduit as the gas holder approaches the lowermost portion of itsvertical movement.

6. A sewage sludge digester comprising a sludge holding tank; means forfeeding sludge into the tank; liquid withdrawal means leading from thetank; means providing a gas conduit leading from the upper interiorportion of the digester and a

